Why is the Clay Brush (set to add) actually Subtracting during stroke? (Sculpting)
The behavior is caused by Autosmooth (Sidebar (N) &gt; Tool &gt; Brush Settings). This setting controls the amount of smoothing that is applied to each brush stroke. In your project the positive displacement of the clay brush is cancelled out by the stronger Autosmooth. The strength of Autosmooth depends both on the factor in the tool settings and the resolution of the mesh. An object will be affected less in its shape the higher the mesh resolution is.
Hence, there are three options to reduce this unintended displacement:

Reduce the strength of Autosmooth.
Increase the strength of the clay brush.
Use a higher number of subdivisions.

The first and second option are the most straight forward solution to the problem. The third option is only listed for completeness, it is likely not desirable to increase the number of subdivisions just to reduce the impact of Autosmooth.